Acoustic horn

ABSTRACT

An acoustic horn having a plurality of spaced longitudinal ribs extending into the horn for reducing the cross-sectional area of the throat thereof to provide a broadband impedance transformation with minimum phase cancellation between an acoustic transducer element and an acoustic transmission medium.

11 1e 1 States Patent 1 1 2 3,852,529 Schafft 1 1 Dec. 3, 1974 1 ACOUSTIC HORN 2,135,610 11/1938 Wcntc 1111/27 R 2,537,141 1 1951 K1 h; ..181/Z7 R [75] Inventor: Hugo Willy Schafft, Des Plames, 111. v [p36 73] Assignee: Motorola, Inc., Franklin Park, 111. P i E i -K thl n H CIaff Assistant Examiner-Douglas W. Olms [22] Flled' Jan. 1973 Attorney, Agent, or Firm-Eugene A. Parsons; Vincent [21] Appl. No.: 322,601 J. Rauner [52 U.s.c1. ..179/i MG, 18l/27R [57] ABSTRACT [51] Int. Cl H04r 1/20 [58] Field of Search 179/] Isl/27R 27 An ac0ust1c horn having a plurahty of spaced 1ong1tuv dinal ribs extending into the horn for reducing the [56] References Cited cross-sectional area of the throat thereof to provide a 1 broadband impedance transformation with minimum UNITED STATES PATENTS phase cancellation between an acoustic transducer e1 2,001,089 5/1935 Blattner 181/27 R ement and an acoustic transmission medium. 2,107,757 2/1938 Kins1ey.1...,. 2,109,012 2/1938 Lawrance 181/27 R 16 e rns, 11 Drawing Figures- PAIENIE'u-B 1 SHEET 1 [IF 2 PRIOR ART PRIOR ART PR/OR ART PAIENIE mac 3:914

sum 2 m 2 cancellations.

ACOUSTIC HORN BACKGROUND I. Field of Invention This invention relates generally to acoustic transduccross-sectional area of the horn in the throat area to provide the horn effect, and the resultant spaces between the ribs provide passages wherein all points on ers, and more particularly to acoustic transducers employing a horn to provide an impedance transformation between a transducer element and an acoustic transmission medium.

2. Prior Art Compression horn type transducers of the prior art comprise a vibrating diaphragm and a tapered horn to provide an'impedance transformation between the diaphragm and the air or" other. acoustic transmission medium. A compression chamber is generally employed between the diaphragm and horn to transfer acoustic energy therebetween. To minimize the distortion produced by the transducer, the area of the diaphragm must be relatively large, and is usually larger than the cross-sectional area of the throat of the horn. This size difference causes acoustic waves from the center of the, diaphragm to reach the throat of the born before vibraphragm. Anotherxsuch system uses a phase correcting plug having a plurality-of v holes, radial slots or annular rings therein for equalizing the path'length between the various portions of the diaphragm and the horn;

' Whereas these techniques provide a way to reduce thephase cancellationeffects in'a' compression "horn, I

multiple horn structures -are.relatively. complex, and

phase cancelling plugs must be built to precision tolerances'. Both techniques result in increased complexity and transducer manufacturing cost. 1

S MMAR FIGS. 5 and 6 taken along the section line E-E of FIG.

It is an objectof the present invention to provide 'a horn foran acoustic transducer.whichminimizes phase It is'anotherfobject of this invention telprovide a horn I 'for art-acoustic transducer that does correction plug. v v Yet another. object of this invention is toprovide a broadband low distortio'nacoustic transducer.

. It is a further object of this invention to.provide a highquality acoustic transducer-that is simple and relanot require a phase tively inexpensive to manufacture.

-.A still'furthe'r object of this invention isito provide a horn having a substantially,Iconstantpath'length between various areas of the diaphragm-and mouth of the horn that can befabricated as azsingle assembly. 1 Inaccordance with a preferred. embodiment of the invention, a horn having longitudinal ribs therein for reducing the cross-sectional area of thehornin the re- 7 gion of the throatthe'reo'f, is'molded from plastic or formed from any suitable material.. The ribs reduce the the driver diaphragm are approximately the same distance away from the throat, thereby substantially minimizing phase cancellation. In addition, the sound waves from the diaphgram travel in a straight line to the mouth of the horn, thereby eliminating the losses caused by the contoured path of prior art phase correcting devices. Finally, the horn can be molded in a single assembly very inexpensively using current plastic molding techniques.

DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a cross-sectional view of a compression horn according to the prior art, and is included to illustrate the problem of phase cancellation in compression horns;

FIG. 2 is a cross-sectional view of a similar horn utilizing a phase correction plug according to the prior art;

FIG. 3 is a cross-sectional view of a horn according to the prior art utilizing another type of phase corrects P g;

FIG- 4 is a cross-sectional view of a horn according to the invention which provides'a substantially constant path length between the diaphragm of thetransducer and the mouthot. the-horn; I FIGS. 4a, 4b and 4c are cross-sectional views taken along line A-A, B-B and C Cof FIG. 4, respectively;

.FIG. Sis an end'view of one embodiment of a horn according to the invention having a center plug to provide for more accurate controlof the cross-sectional area of the passageway between the longitudinal ribs;

FIG. 6 isa cross-sectional view of the horn of FIG. '5 taken along the sectionlineD-D of FIG. 5

FIG. 7 is a cross-sectional view of the horn shown in 6; and

- FIG. .8 isan end view of the throat end of the horn shown in FIG. "5.

DETAILED DESCRIPTION I 'Refe'rrin'gto FIG. Lthere is-showna diagram of a basic compression horn according to'the prior art. A driver 10, which may be a magnetic orpiezoel ectric transducer element, or other acoustic translating means,iis coupled 'toa cone or diaphragm 14 by means of'a'suitable coupling element 12. The 'cone l4 forms one wall of a compression chamber 16 to which a horn 18 having a mouth 20 and a throat-22 is attached.

- In operation, the driver 10 applies acoustic vibrations to the cone l4 viathe'coupling means 12. The cone l 4 serves as an impedance matching device-between the throat 22 of the horn and thedriver 10. Some degree of impedance matching'is' necessary, particularly for piezoelectric drivers which provide high force and low displacement, in order toachieve proper coupling of energy to'the throat of the horn. In order to minimize distortion, thecone should be made as large as possible, and should be stiff enough not to break up into res-. onant modes within the operating frequency range. The

cone 14, which moves in response to the force applied thereto by driver 10, causes the air in the compression chamber 16 to be; compressed in accordance with the force applied by the driver 10, and the pressure variadistance D between the throat 22 of the horn and the edge of the cone 14 causes thepressure waves generated' by different areas of the cone 18 to reach the throat 22 of the horn 18 at different times with respect to each other, thereby causing phase cancellations.

These phase cancellations manifest themselves as periodic minima in the frequency response curve of the transducer and seriously limit the high frequency operation of the horn.

Referring to FIG. 2, there is shown a horn similar to the horn of FIG. -1 having a phase correction plug therein to reduce the phase cancellation problem. The

transducer of FIG. 2 is similar to the transducer of FIG.

1 and has a driver 30, a coupling means 32,'a cone or diaphragm 34,;a compression chamber 3,6and a horn 38 having a throat 42 and amouth 40. The aforementioned components of thetransducer are similar to analogous components'of the transducer of FIG. 1. In thetransducer of FIG. 2,'a phase correction plug 44 is interposed between the compression chamber 36 'and the throat 42 of horn 38. The phase correctionplug includes a series of passages 46 between the compression chamber 36 and the throat 42.The passages are designed to provide a substantially constant path length between the compression chamber 36 and throat 42 to cause the pressure waves from various portions of the diaphragm 34 to arrive at the throat 42 atfapproxi mately the'same time, thereby minimizing phase'cancording to the invention. In this embodiment, a piezoelectric driver 70, which is supported by a support member 71, drives a cone or diaphragm 74 which couples acoustic energy to a compression chamber 76. The diaphragm-74 is attached to a frame member for support and forms one wall of the compression chamber 76. The horn 78, which alsohas a throat 82 and a mouth 80, is coupled to receive acoustic energy from the compression chamber 76.-The throat end of the horn 78 has a smaller diameter than the diameter of the frame in this embodiment, however it should be noted that the throat end of the horn' and the frame may have the same diameter and still fall withi'nithe scope of the invention.

In order for a horn to perform the function of providing an impedance transformation between'a' transducer element and air or othertransmission medium, it is necessary for the cross-sectional area of the horn to vary along the longitudinal dimension of the horn. Generally the throat of a horn has a smaller area than the mouth.

The conventional techniques of reducing the area at the throat by reducing the throat diarneterhavecaused the problems described' previously in this specification.

In thehorn 78 of the instant invention, the area at the throat is reduced by the plurality of ribs 84 mounted within a tubular member which formsthe horn. in this embodiment, the ribs have a' nonuniform crossf sectional area and are positioned with the ends having the larger cross-sectional area toward the throat of the horn. The ribs 84 reduce the throat area of the horn, but allow the diameter of the throat area of the horn to remain substantially similar to the diameter of the diaphragm or cone 74. This effectively spreads the throat area 82 over the surface of the diaphragm 74 to provide 'cellationsLf-Although this technique provides some;

' more, the plug must be made to close tolerance specifications and the distance between the cone and the plug iscritical. He: J I, Referring to,FIG.. 3, ,ther e: is shown a cross-sectional fview'of a horn'utilizinjg another type of phase correc- 'tion de'vice. ln this embodiment, a driver 50 drivesa cone 54 which-in turn appliesjenergy to a horn 58 g through an annular -port62. A phase correctingplug64 ismounted with'in thehorn 58 ,arid'forms alsecond horn 68. ln this type of horn, the largerhorn 58 is driven primarily bythe edge ofconej-54a'nd the smaller horn68 a circular 'diaphragm is used to drive a circular horn,

a substantially constant distance, or path length, between each portion of the diaphragm area and the mouth of the horn to minimize phase cancellations within the operating range of the horn,

FIG. 4ashows the cross-sectional area of the throat 82 of the horn 78 at point A-Axln this embodiment,

however any cross-sectional shapeiri'c'luding square or rectangular shapes'may be used. The diameter of the throat area is substantiallysimilar to the diameter'of the diaphragm74, and six' triangular ribs 84 extend radially into the throat area toprovide the star shaped re duced throat area, 82. The area of the throat 82 maybe made similar to the area'of throats 22 and 42 of the horns 'of FIGS. l and.2,re'spectively; 'togprovide animreceives its energypriinari'ly.fromthe center of the I tween the throat 'ofeach horn and itsrespective'portion of the cone 54 are approximately-"the same and phase cone 54 through passageways 66. The distances becancellations-are minimized. However,-this scheme s'uf-v fers from the'j'sam'e cost andcomplxity limitations as the. scheme s'hown'in FIG. 2.

' "Referring to ,FIG..'4,-there-'isshown a cross sectional view of a transducer employing a horn 78-having a plurality of ribs 84 for reducingthe cross-sectionalarea of the horn, particularly inthe-thr'oat area thereof, ac-

" pedance transformation similar to; th t ransf 'ma id provided by orn-sot the-pridf t t FIG. 4bshows thecross sectionalareaof the horn at a point B-'B approximately" "midway between the throat and the mo'uth'tl'iereof. In this embodiment, the diameterof the horn ir 1 FlG.-4b is ap'proitimatelyi the same as the diameter of the hem at FIG. 421: However, the area within the hornin FlGfltb is' substantially larger than the area'of the throat'82 due to areduction in the cross-sectional area of -the ribs 84 at point B- --B.

FIG. shows a cross-section the "mouth 80, at point C C. The ribs 8 4 'shown-in'FlG, 40 have a substantially reduced cross-sectional area and the area of themou-th of the hornis determined primarily by the diameter thereof.

taper of the ribs can be chosen to provide a linearly tapered horn, a hyperbolic horn, an exponential horn or any horn taper obtainable with conventional horns. Note also that the ribs need not extend the entire longitudinal length of the horn, but may end before reaching the mouth 80 of the horn, beyond which point the cross-sectional area of the horn can be increased by simply increasing the size of the horn. In addition, all of the ribs need not be tapered, as in the case of rectangular horns wherein it may be desirable to taper some ribs and not others.

Referring to FIG. 6, there is shown another embodiment of the horn according to the invention. For purposes of clarity, no transducer has been shown in FIGS. 5-8, and only the horn has been shown, however, it should be understood that the horn shown in FIGS. 5-8 may be driven by any suitable'transducer, such as, for example, the piezoelectric transducer shown in FIG. 4. The horn shown in FIG. 6 includes a throat end 90 and a mouth end 92. A plurality of ribs 94 are employed to reduce the crosssectional area of the horn, particularly in the throat area. A central plug 96 is positioned along a central axis of the horn and is supported by the ribs 94. The plug 96 provides a means for more accurately 94, the central plug 96 thereby plugging the center portion of the throat area to define a series of passages 98 between the ribs 94 and the plug96. Because the passages 98 are shorter along the radial dimension than the individual arms of the starshaped cross-sectional area. 82 shown in FIG. 4a, for a' given cross-sectional area, the width of each of the passages 98 is greater than the width of each of the radial arms of the star shaped cross-sectional area-thereby making the throat area more independent of width tolerances'and providing better control of the throat area. .Since'thethroat area is controlled by the thickness of the ribs 94 and by the diameter of the central plug 96, the size of the' ribs 94 and the central plug 96 can be chosen toprovi'dean op area.

horn of FIG. 4 to provide increased lightness while maintaining simplicity of construction.

Although many particular configurations of a horn according to the instant invention are possible, it should be noted that any horn utilizing a plurality of ribs to reduce the size of the throat area while maintaining a substantially constant path length between the transducer diaphragm and the mouth or throat of the horn fall within the scope of the invention. Furthermore, the horn of the instant invention may be used with a translating element for either reproducing or picking up acoustic waves from air, water or other acoustic transmission medium.

In summary, the horn of theinstant invention provides a simple, inexpensive way to produce ahorn type transducer having a minimum of phase cancellations. This is achieved by a structure wherein all points on the diaphragm are substantially the same distance away from the mouth and throat, and wherein the sound from the diaphragm travels in a substantially straight line to the mouth of the horn, thereby further extending the high frequency response of the transducer.

I claim:

1. A horn for providing animpedance transformation between an acoustic translating means and an acoustic transmission medium comprising: an elongated tube member having a throat end connectible to said acoustic translating means and a mouth end for transferring acoustic energy between-said horn and said acoustic transmission medium, said tube member having a plurality of elongated ribs positionedlongitudinally within the interior of said tube member, each rib extending from an interior surface of said tube member into the space enclosed thereby to reduce the cross-sectional area, thereof, adjacent ones of said ribs being spaced substantiallyan equal distance from each other within phase cancellations withinthe operating range of said As can. be seen from FIG. 6, the combination of the ribs 94 and the central plug 96 define a smoothly increasing cross-sectional area between the throat portion and the mouth portion of the horn. There is no .abrupt change in cross-sectional area at the point of separation 100 between the rib 94 and the central plug 96. The diameter of the central plug 96, shown in FIG. 6 is substantially constant along itslongitudinal dimension,.however, the diameter may be varied along with the cross-sectional area of the ribs toprovide any desired taper such as, for example, maintaining the crosssectional area of the slots 98 within the throat area 90- hollow construction, similar to. the construction of the horn. I

2. A horn as recited in claim 1 further including a plug positioned along a central axis thereof, said plug being supported at the throat end by said ribs.

3. A horn as recited in claim 1 wherein at least one of said ribs is tapered and positioned with the end of the rib having the larger cross-sectional area toward the throat end of said tube member. s

4. A horn as recited in claim 1 wherein said integral to said tube member.

5. A horn type acoustic transducer including-in combination; an acoustic transducer element, a frame member, a diaphragm attached to said frame member to define a compression chamber having an opening therein, said diaphgram forming a wall of said compression chamber, means coupling said transducer element and said diaphragm for transferring vibrational energy therebetween, and a horn having a mouth end and a throat end, said throat end communicating with the ribs are opening in said chamber, wherein said horn has spaced longitudinal ribs on the interior surface thereof for re- 1 ducing the cross-sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and defining a plurality of similar longitudinally extending passages to effectively spread the area of said throat over the surface I ducer element.

of said diaphragm to provide substantially equal path lengths for said passages' between said diaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.

6'. A transducer as recited in claim further including a plug supported by said ribs, said plug and said ribs defining a plurality of passages therebe'tween, each of said I passages having a cross-sectional area which increases smoothly toward the mouth of the horn along a longitu-' dinal axis thereof.

7. A transducer as recited in claim 5 wherein each of said passages is substantially straight.

8. .A transducer as recited in claim 6 wherein said ,.aco.tistic transducer element is a piezoelectric transforming'a wall of said chamber,.-and a horn having a mouth end anda throatend,. -said throat'end communiover the surface of said diaphragm to provide substantially equal path lengths for said passages between saiddiaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.

11. An acoustic transformation device as recited in claim 10 further including an elongated plug'positioned longitudinally within said horn and supported at the throat end by said ribs, said plug co-operating with said ribs to define said passages.

12. An acoustic impedance transformation device asrecited in claim 10 wherein each of said passages is substantially straight.

13. An acoustic impedance transformation device as recited in claim 12 wherein at least one of said ribs is tapered and positioned with the end of the rib having eating with the openingin said chamber, wherein said horn has spaced longitudinal'ribs on the, interior surface thereof for reducing the cross sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and definingaplurality' of similar longitudinally extending passages toeffectively spread the area of's'aid throat the larger cross-sectional area positioned toward the throat of said horn.

14. An acoustic impedance transformation device as recited in claim 12 wherein said ribs areintegral to said horn.

15. An acoustic impedance transformation device as recited in claim 14. wherein said horn has asubsta tially circular cross-section and'wherein said ribs ex- .tend radially 'into'the horn from the interior surface thereof.

16. An acoustic impedancetransformation device as recited in claim 15 wherein'said'diaphragm isxcircular and wherein the diameter of said throat is substantially similar to the diameter of said diaphragm. 

1. A horn for providing an impedance transformation between an acoustic translating means and an acoustic transmission medium comprising: an elongated tube member having a throat end connectible to said acoustic translating means and a mouth end for transferring acoustic energy between said horn and said acoustic transmission medium, said tube member having a plurality of elongated ribs positioned longitudinally within the interior of said tube member, each rib extending from an interior surface of said tube member into the space enclosed thereby to reduce the cross-sectional area, thereof, adjacent ones of said ribs being spaced substantially an equal distance from each other within said tube member and defining therebetween longitudinally extending passages having substantially equal lengths between said throat and mouth end to provide maximum high frequency response and to minimize phase cancellations within the operating range of said horn.
 2. A horn as recited in claim 1 further including a plug positioned along a central axis thereof, said plug being supported at the throat end by said ribs.
 3. A horn as recited in claim 1 wherein at least one of said ribs is tapered and positioned with the end of the rib having the larger cross-sectional area toward the throat end of said tube member.
 4. A horn as recited in claim 1 wherein said ribs are integral to said tube member.
 5. A horN type acoustic transducer including in combination; an acoustic transducer element, a frame member, a diaphragm attached to said frame member to define a compression chamber having an opening therein, said diaphgram forming a wall of said compression chamber, means coupling said transducer element and said diaphragm for transferring vibrational energy therebetween, and a horn having a mouth end and a throat end, said throat end communicating with the opening in said chamber, wherein said horn has spaced longitudinal ribs on the interior surface thereof for reducing the cross-sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and defining a plurality of similar longitudinally extending passages to effectively spread the area of said throat over the surface of said diaphragm to provide substantially equal path lengths for said passages between said diaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.
 6. A transducer as recited in claim 5 further including a plug supported by said ribs, said plug and said ribs defining a plurality of passages therebetween, each of said passages having a cross-sectional area which increases smoothly toward the mouth of the horn along a longitudinal axis thereof.
 7. A transducer as recited in claim 5 wherein each of said passages is substantially straight.
 8. A transducer as recited in claim 6 wherein said acoustic transducer element is a piezoelectric transducer element.
 9. A transducer as recited in claim 6 wherein said acoustic transducer element is a magnetic transducer element.
 10. An acoustic transformation device including in combination; a frame member, a diaphragm connectible to an acoustic transducer element to be driven thereby attached to said frame member to define a chamber having an opening therein, said diaphragm forming a wall of said chamber, and a horn having a mouth end and a throat end, said throat end communicating with the opening in said chamber, wherein said horn has spaced longitudinal ribs on the interior surface thereof for reducing the cross-sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and defining a plurality of similar longitudinally extending passages to effectively spread the area of said throat over the surface of said diaphragm to provide substantially equal path lengths for said passages between said diaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.
 11. An acoustic transformation device as recited in claim 10 further including an elongated plug positioned longitudinally within said horn and supported at the throat end by said ribs, said plug co-operating with said ribs to define said passages.
 12. An acoustic impedance transformation device as recited in claim 10 wherein each of said passages is substantially straight.
 13. An acoustic impedance transformation device as recited in claim 12 wherein at least one of said ribs is tapered and positioned with the end of the rib having the larger cross-sectional area positioned toward the throat of said horn.
 14. An acoustic impedance transformation device as recited in claim 12 wherein said ribs are integral to said horn.
 15. An acoustic impedance transformation device as recited in claim 14 wherein said horn has a substantially circular cross-section and wherein said ribs extend radially into the horn from the interior surface thereof.
 16. An acoustic impedance transformation device as recited in claim 15 wherein said diaphragm is circular and wherein the diameter of said throat is substantially similar to the diameter of said diaphragm. 